Inhibitors of Platelet Adhesion

Lipoprotein(a), platelet function and cardiovascular disease

Lipoprotein(a) (Lp(a)) is associated with atherothrombosis through several mechanisms, including putative antifibrinolytic properties. However, genetic association studies have not demonstrated an association between high plasma levels of Lp(a) and the risk of venous thromboembolism, and studies in patients with highly elevated Lp(a) levels have shown that Lp(a) lowering does not modify the clotting properties of plasma ex vivo. Lp(a) can interact with several platelet receptors, providing biological plausibility for a pro-aggregatory effect. Observational clinical studies suggest that elevated plasma Lp(a) concentrations are associated with worse long-term outcomes in patients undergoing revascularization. Furthermore, in these patients, those with elevated plasma Lp(a) levels derive more benefit from prolonged dual antiplatelet therapy than those with normal Lp(a) levels. The ASPREE trial in healthy older individuals treated with aspirin showed a reduction in ischaemic events in those who had a single-nucleotide polymorphism in LPA that is associated with elevated Lp(a) levels in plasma, without an increase in bleeding events. In this Review, we re-examine the role of Lp(a) in the regulation of platelet function and suggest areas of research to define further the clinical relevance to cardiovascular disease of the observed associations between Lp(a) and platelet function.

Platelet Glycoprotein-Ib (GPIb) May Serve as a Bridge between Type 2 Diabetes Mellitus (T2DM) and Atherosclerosis, Making It a Potential Target for Antiplatelet Agents in T2DM Patients

Type 2 diabetes mellitus (T2DM) is a persistent metabolic condition that contributes to the development of cardiovascular diseases. Numerous studies have provided evidence that individuals with T2DM are at a greater risk of developing cardiovascular diseases, typically two to four times more likely than those without T2DM, mainly due to an increased risk of atherosclerosis. The rupture of an atherosclerotic plaque leading to pathological thrombosis is commonly recognized as a significant factor in advancing cardiovascular diseases caused by TD2M, with platelets inducing the impact of plaque rupture in established atherosclerosis and predisposing to the primary expansion of atherosclerosis. Studies suggest that individuals with T2DM have platelets that display higher baseline activation and reactivity than those without the condition. The expression enhancement of several platelet receptors is known to regulate platelet activation signaling, including platelet glycoprotein-Ib (GPIb). Furthermore, the high expression of platelet GP1b has been reported to increase the risk of platelet adhesion, platelet-leucocyte interaction, and thrombo-inflammatory pathology. However, the study exploring the role of GP1b in promoting platelet activation-induced cardiovascular diseases in T2DM patients is still limited. Therefore, we summarize the important findings regarding pathophysiological continuity between T2DM, platelet GPIb, and atherosclerosis and highlight the potential therapy targeting GPIb as a novel antiplatelet agent for preventing further cardiovascular incidents in TD2M patients.

Posttranslational modifications of platelet adhesion receptors

Platelets play a key role in normal hemostasis, whereas pathological platelet adhesion is involved in various cardiovascular events. The underlying cause in cardiovascular events involves plaque rupture leading to subsequent platelet adhesion, activation, release, and eventual thrombosis. Traditional antithrombotic drugs often target the signal transduction process of platelet adhesion receptors by influencing the synthesis of some key molecules, and their effects are limited. Posttranslational modifications (PTMs) of platelet adhesion receptors increase the functional diversity of the receptors and affect platelet physiological and pathological processes. Antithrombotic drugs targeting PTMs of platelet adhesion receptors may represent a new therapeutic idea. In this review, various PTMs, including phosphorylation, glycosylation, ubiquitination, nitrosylation, methylation, lipidation, and proteolysis, of three platelet adhesion receptors, glycoprotein Ib-IX-V (GPIb-IX-V), glycoprotein VI (GPVI), and integrin α_IIbβ_3, are reviewed. It is important to comprehensively understand the PTMs process of platelet adhesion receptors.

Changes in MicroRNA Expression Level of Circulating Platelets Contribute to Platelet Defect After Cardiopulmonary Bypass

OBJECTIVES

Platelet defect mechanisms after cardiopulmonary bypass remain unclear. Our hypothesis microRNA expressions in circulating platelets significantly change between pre and post cardiopulmonary bypass, and consequent messenger RNA and protein expression level alterations cause postcardiopulmonary bypass platelet defect.

DESIGN

Single-center prospective observational study.

SETTING

Operating room of Kyoto Prefectural University of Medicine.

PATIENTS

Twenty-five adult patients scheduled for elective cardiac surgeries under cardiopulmonary bypass.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

In the initial phase, changes in microRNA expression between pre and post cardiopulmonary bypass underwent next generation sequencing analysis (10 patients). Based on the results, we focused on changes in mir-10b and mir-96, which regulate glycoprotein 1b and vesicle-associated membrane protein 8, respectively, and followed them until messenger RNA and protein syntheses (15 patients) using quantitative polymerase chain reaction and Western blotting. Seven microRNAs including mir-10b and mir-96 exhibited significant differences in the initial phase. In the subsequent phase, mir-10b-5p and mir-96-5p overexpressions were confirmed, and glycoprotein 1b and vesicle-associated membrane protein 8 messenger RNA levels were significantly decreased after cardiopulmonary bypass: fold differences (95% CI): mir-10b-5p: 1.35 (1.05-2.85), p value equals to 0.01; mir-96-5p: 1.59 (1.06-2.13), p value equals to 0.03; glycoprotein 1b messenger RNA: 0.46 (0.32-0.60), p value of less than 0.001; and vesicle-associated membrane protein messenger RNA: 0.70 (0.56-0.84), p value of less than 0.001. Glycoprotein 1b and vesicle-associated membrane protein 8 were also significantly decreased after cardiopulmonary bypass: glycoprotein 1b: 82.6% (71.3-93.8%), p value equals to 0.005; vesicle-associated membrane protein 8: 79.0% (70.7-82.3%), p value of less than 0.001.

CONCLUSIONS

Expressions of several microRNAs in circulating platelets significantly changed between pre and post cardiopulmonary bypass. Overexpressions of mir-10b and mir-96 decreased glycoprotein 1b and vesicle-associated membrane protein 8 messenger RNA as well as protein, possibly causing platelet defect after cardiopulmonary bypass.

Investigating the Effect of Demographics, Clinical Characteristics, and Polymorphism of MDR-1, CYP1A2, CYP3A4, and CYP3A5 on Clopidogrel Resistance

Clopidogrel is an antiplatelet agent that is indicated for cardiovascular emergencies and procedures. The drug, however, is subject to response variability leading to therapy resistance. In this research, we explored the demographic, clinical, and genetic factors associated with clopidogrel resistance. Data analysis among our 280 subjects receiving clopidogrel showed some risk factors that are significantly associated with clopidogrel resistance compared with responders. Those were: female sex (P = 0.021), advanced age (P = 0.011), obesity (P = 0.002), and higher body mass index (P = 0.008) and higher platelets count (P = 0.002). However, known polymorphisms of MDR-1, CYP1A2, CYP3A4, and CYP3A5 were not associated with treatment resistance when compared to responders to clopidogrel therapy. Knowledge about such risk factors might provide recommendation in the future about starting doses or monitoring recommendations.

Changes in platelet Bax levels contribute to impaired platelet response to thrombin after cardiopulmonary bypass: prospective observational clinical and laboratory investigations

Background

Anucleate platelets can undergo apoptosis in response to various stimuli, as do nucleated cells. Cardiopulmonary bypass (CPB) causes platelet dysfunction and can also activate platelet apoptotic pathways. We therefore evaluated time-dependent changes in blood platelet Bax (a pro-apoptotic molecule) levels and platelet dysfunction after cardiac surgery.

Methods

We assessed blood samples obtained from subjects having on-pump or off-pump coronary artery bypass graft surgery ( n =20 each). We also evaluated the in vitro effects of platelet Bax increase in eight healthy volunteers.

Results

Thrombin-induced platelet calcium mobilisation and platelet-surface glycoprotein Ib (GPIb) expression were lowest at weaning from CPB and did not recover on postoperative day one. On-pump surgery increased platelet expression of Bax, especially the oligomerised form, along with translocation of Bax from the cytosol to mitochondria and platelet-surface tumour necrosis factor-alpha (TNF-α)-converting enzyme (TACE) expression. In contrast, mitochondrial cytochrome c expression was reduced. While similar in direction, the magnitude of the observed changes was smaller in patients having off-pump surgery. In vitro , a cell-permeable Bax peptide increased platelet Bax expression to the same extent seen during bypass and produced similar platelet changes. These apoptotic-like changes were largely reversed by Bcl-xL pre-administration, and were completely reversed by combined application of inhibitors that stabilise outer mitochondrial membrane permeability and TACE.

Conclusions

CPB increases platelet Bax expression, which contributes to reduced platelet-surface GPIb expression and thrombin-induced platelet calcium changes. These changes in platelet apoptotic signalling might contribute to platelet dysfunction after CPB.

Clinical trial registration

UMIN Clinical Trials Registry (number UMIN000006033).

Common Variant in Glycoprotein Ia Increases Long-Term Adverse Events Risk After Coronary Artery Bypass Graft Surgery

BACKGROUND

This study was aimed to investigate the clinical relevance between glycoprotein Ia (GPIA) rs1126643C/T polymorphism and the outcome of coronary artery disease after coronary artery bypass graft (CABG) surgery and explore the involved potential mechanisms.

METHODS AND RESULTS

We genotyped GPIA rs1126643 polymorphism of 1592 patients who underwent CABG and followed up for a median period of 72.8 months. Patients who are GPIA rs1126643 T-allele carriers have a higher major adverse cardiac or cerebrovascular events risk post-CABG than those who are CC homozygotes (hazard ratio [HR]=1.29; P=0.022). The clinical association between the risk allele (T) carriage and major adverse cardiac or cerebrovascular events was confirmed in another cohort study, which included 646 CABG patients from various health centers across China. Meanwhile, rs1126643 T allele was also linked with increased risk of major adverse cardiac or cerebrovascular events (HR=1.73; P=0.019). To explore the underlying mechanisms, we prospectively recruited 131 coronary artery disease patients, assessed their platelet aggregation function, and focused on detecting their GPIA mRNA level and protein expression. Results showed that patients with rs1126643 T allele have elevated platelet aggregation activity (P=0.029) when protein expression is increased (P<0.001) and not affected by glycoprotein Ia mRNA level.

CONCLUSIONS

The synonymous common variant, GPIA rs1126643, increases the long-term adverse events risk of CABG by augmenting GPIa protein expression and enhancing platelet aggregation function. This finding can serve as the implication of improving secondary prevention of CABG patients.

The role of platelets in coagulation dysfunction in xenotransplantation, and therapeutic options

Xenotransplantation could resolve the increasing discrepancy between the availability of deceased human donor organs and the demand for transplantation. Most advances in this field have resulted from the introduction of genetically engineered pigs, e.g., α1,3-galactosyltransferase gene-knockout (GTKO) pigs transgenic for one or more human complement-regulatory proteins (e.g., CD55, CD46, CD59). Failure of these grafts has not been associated with the classical features of acute humoral xenograft rejection, but with the development of thrombotic microangiopathy in the graft and/or consumptive coagulopathy in the recipient. Although the precise mechanisms of coagulation dysregulation remain unclear, molecular incompatibilities between primate coagulation factors and pig natural anticoagulants exacerbate the thrombotic state within the xenograft vasculature. Platelets play a crucial role in thrombosis and contribute to the coagulation disorder in xenotransplantation. They are therefore important targets if this barrier is to be overcome. Further genetic manipulation of the organ-source pigs, such as pigs that express one or more coagulation-regulatory genes (e.g., thrombomodulin, endothelial protein C receptor, tissue factor pathway inhibitor, CD39), is anticipated to inhibit platelet activation and the generation of thrombus. In addition, adjunctive pharmacologic anti-platelet therapy may be required. The genetic manipulations that are currently being tested are reviewed, as are the potential pharmacologic agents that may prove beneficial.

Local sustained delivery of acetylsalicylic acid via hybrid stent with biodegradable nanofibers reduces adhesion of blood cells and promotes reendothelialization of the denuded artery

Incomplete endothelialization, blood cell adhesion to vascular stents, and inflammation of arteries can result in acute stent thromboses. The systemic administration of acetylsalicylic acid decreases endothelial dysfunction, potentially reducing thrombus, enhancing vasodilatation, and inhibiting the progression of atherosclerosis; but, this is weakened by upper gastrointestinal bleeding. This study proposes a hybrid stent with biodegradable nanofibers, for the local, sustained delivery of acetylsalicylic acid to injured artery walls. Biodegradable nanofibers are prepared by first dissolving poly(D,L)-lactide-co-glycolide and acetylsalicylic acid in 1,1,1,3,3,3-hexafluoro-2-propanol. The solution is then electrospun into nanofibrous tubes, which are then mounted onto commercially available bare-metal stents. In vitro release rates of pharmaceuticals from nanofibers are characterized using an elution method, and a highperformance liquid chromatography assay. The experimental results suggest that biodegradable nanofibers release high concentrations of acetylsalicylic acid for three weeks. The in vivo efficacy of local delivery of acetylsalicylic acid in reducing platelet and monocyte adhesion, and the minimum tissue inflammatory reaction caused by the hybrid stents in treating denuded rabbit arteries, are documented. The proposed hybrid stent, with biodegradable acetylsalicylic acid-loaded nanofibers, substantially contributed to local, sustained delivery of drugs to promote re-endothelialization and reduce thrombogenicity in the injured artery. The stents may have potential applications in the local delivery of cardiovascular drugs. Furthermore, the use of hybrid stents with acetylsalicylic acid-loaded nanofibers that have high drug loadings may provide insight into the treatment of patients with high risk of acute stent thromboses.

Influence of red algal sulfated polysaccharides on blood coagulation and platelets activation in vitro

The influence of sulfated polysaccharides (λ-, κ-, and κ/β-carrageenan and porphyran) - on platelet activation was studied. Carrageenans were much weaker inhibitors of a coagulation process than heparin, while porphyran had not that effect. Results of the aPTT and PT assays suppose that carrageenans affected mostly intrinsic pathway of coagulation, while their effect on the extrinsic pathway is extremely low (λ and κ/β) or absent (κ, LMW derivative of κ-carrageenan). λ-Carrageenan was the most potent anticoagulant agent in TT, aPTT, PT, and anti-factor Xa activity. This sample was also the strongest inhibitor of collagen-induced platelet aggregation in PRP. Generally, the correlation of anticoagulant and antithrombotic action in PRP is preserved for carrageenans but not for heparin. Carrageenans and porphyran affected platelet adhesion to collagen by influencing glycoprotein VI. Low molecular weight κ-carrageenan had a similar effect on platelet adhesion mediated with both major collagen receptors: integrin α2 β1 and glycoprotein VI as native polysaccharide had. Carrageenans resulted in activation of platelets under platelet adhesion mediated by integrin αIIb β3 with less degree than heparin. The least sulfated κ/β-carrageenan that possessed an inhibiting effect on thrombin- and collagen-induced aggregation of washed platelets and on the PT test but it had no significant effect on TT was the weakest promoter of integrin αIIb β3 mediated platelet activation. In summary, our study showed that the polysaccharide action was complex, since it depended on its molecular mass, sulfation degree, and monosaccharide contents (3,6-anhydrogalactose).

Targeting von Willebrand factor as a novel anti-platelet therapy; application of ARC1779, an Anti-vWF aptamer, against thrombotic risk

Excessive activation of platelets is a causative factor for thrombotic diseases such as acute coronary syndrome or stroke, and various anti-platelet drugs were developed. Aspirin and clopidogrel have been used as gold standards for anti-platelet therapies, however, their clinical limitations including bleeding problem have increased the demand driving development of novel anti-platelet drugs with new targets. Among several activating pathways leading to platelet aggregation, the interaction between von Willebrand factor (vWF) and glycoprotein Ib, which mainly occurs under high shear stress in arterioles, is recently suggested to be a new promising target. The anti-thrombotic efficacy of anti-vWF agents, such as ARC1779, has been proved in several preclinical and clinical studies. Here, we will discuss the potential benefits of targeting vWF as a novel antiplatelet therapy, providing an insight into the role of vWF in increased thrombotic risk.

Novel antiplatelet activity of protocatechuic acid through the inhibition of high shear stress-induced platelet aggregation

Bleeding is the most common and serious adverse effect of currently available antiplatelet drugs. Many efforts are being made to develop novel antithrombotic agents without bleeding risks. Shear stress-induced platelet aggregation (SIPA), which occurs under abnormally high shear stress, plays a crucial role in the development of arterial thrombotic diseases. Here, we demonstrate that protocatechuic acid (PCA), a bioactive phytochemical from Lonicera (honeysuckle) flowers, selectively and potently inhibits high shear (>10,000 s(-1))-induced platelet aggregation. In isolated human platelets, PCA decreased SIPA and attenuated accompanying platelet activation, including intracellular calcium mobilization, granule secretion, and adhesion receptor expression. The anti-SIPA effect of PCA was mediated through blockade of von Willebrand factor binding to activated glycoprotein Ib, a primary and initial event for the accomplishment of SIPA. Conspicuously, PCA did not inhibit platelet aggregation induced by other endogenous agonists like collagen, thrombin, or ADP that are important in both pathological thrombosis and normal hemostasis. Antithrombotic effects of PCA were confirmed in vivo in a rat arterial thrombosis model, where PCA significantly delayed the arterial occlusion induced by FeCl(3). Of particular note, PCA did not increase bleeding times in a rat tail transection model, whereas conventional antiplatelet drugs, aspirin, and clopidogrel substantially prolonged it. Collectively, these results suggest that PCA may be a novel antiplatelet agent that can prevent thrombosis without increasing bleeding risks.

Which anti-platelet therapies might be beneficial in xenotransplantation?

Xenotransplantation could provide an unlimited and elective supply of grafts, once mechanisms of graft loss and vascular injury are better understood. The development of α-1,3-galactosyltransferase gene-knockout (GalT-KO) swine with the removal of a dominant xeno-antigen has been an important advance; however, delayed xenograft and acute vascular reaction in GalT-KO animals persist. These occur, at least in part, because of humoral reactions that result in vascular injury. Intrinsic molecular incompatibilities in the regulation of blood clotting and extracellular nucleotide homeostasis between discordant species may also predispose to thrombophilia within the vasculature of xenografts. Although limited benefits have been achieved with currently available pharmacological anti-thrombotics and anti-coagulants, the highly complex mechanisms of platelet activation and thrombosis in xenograft rejection also require potent immunosuppressive interventions. We will focus on recent thromboregulatory approaches while elucidating appropriate anti-platelet mechanisms. We will discuss potential benefits of additional anti-thrombotic interventions that are possible in transgenic swine and review recent developments in pharmacological anti-platelet therapy.

CD40-1C>T polymorphism (rs1883832) is associated with brain vessel reocclusion after fibrinolysis in ischemic stroke

AIMS

To find genetic predictors of reocclusion after successful fibrinolytic therapy during the acute phase of ischemic stroke.

PATIENTS & METHODS

This was a case-case prospective study analyzing 236 polymorphisms in a cohort of 222 patients treated with tissue plasminogen activator, from which 16 patients suffered a reocclusion event (7.2%). A predictive scale was generated using independent polymorphisms with a dominant/recessive model and tandem occlusion, weighted by their beta-coefficients in logistic regression.

RESULTS

Using a dominant/recessive model, the rs1800801 SNP from the MGP gene (odds ratio [OR]: 15.25; 95% CI: 2.23-104.46; adjusted p = 0.006) and the rs1883832 SNP from CD40 gene (OR: 0.077; 95% CI: 0.009-0.66; adjusted p = 0.019) were independently associated with reocclusion after logistic regression adjustment by clinical predictors. In an additive model, only the rs1883832 SNP (OR: 4.43; 95% CI: 1.62-12.15; adjusted p = 0.004) was related to reocclusion occurrence. The predictive model that was generated stratified the reocclusion risk from less than 1% to more than 70%. Reocclusions were associated with neurological worsening at 24 h (patients with reocclusion: 26.7%, versus patients without reocclusion: 4.9%; p = 0.002), as it was seen for MGP -7A>G (AA: 17.2% vs AG+GG: 4.5%; p = 0.027), but not for CD40 1C>T (CC: 4.5% vs CT+TT: 7.7%; p = 0.565). There was an association between CD40 -1C>T genotype and CD40 transcriptional activity in peripheral blood mononuclear cells (median expression values TT: 65.75%, CT: 70.80%, CC: 96.00%; p = 0.023). However, CD40 soluble fraction was not a useful biomarker of reocclusion status.

CONCLUSION

An association was found between MGP -7A>G and CD40 -1C>T polymorphisms, and reocclusion risk. The predictive scale that was generated permits the stratification of patients by their reocclusion risk with higher accuracy than clinical parameters alone.